Safe and effective immunosuppressive agents are required for the treatment of patients suffering from autoimmune disorders and for recipients of transplanted organs or tissues. For instance, in the absence of an effective immunosuppressive agent, patients often reject a transplanted organ, sometimes with fatal consequences. The immunosuppressive agent must block the immune response, but must also be sufficiently well tolerated by the body to permit chronic application. For instance, one compound with immunosuppressive activity, FK506, has been used to prevent rejection of transplanted livers. However, severe kidney toxicity has been observed in patients receiving FK506, in some cases requiring kidney transplant following the liver transplant.
Research aimed at discovering new immunosuppressive agents has been hampered by the lack of information about precise molecular mechanisms of the immune response. As a result, random screening of compounds has accounted for a substantial share of research efforts aimed at identifying new immunosuppressive drugs. More recently, "structure-based" approaches to drug design have been attempted. For example, compounds designed to bind to the protein FKBP, one of the cellular targets of FK506, were synthesized as candidate immunosuppressive agents. Those efforts were unfortunately doomed by the lack of understanding of the actual molecular mechanism of immunosuppression mediated by FK506. It is now known that FKS506 binds in a complex with two proteins, FKBP and calcineurin. FK506's immunosuppressive effects are due to the inhibition of calcineurin in T cells. However, since calcineurin is present and important in other cells, FK506 affects other cells and tissues leading to undesired effects.
Meanwhile, independent efforts have led to the identification of a protein tyrosine kinase, ZAP-70, as a critical mediator of the immune response. Blocking the biological function of ZAP-70 will lead to immunosuppression. Unfortunately, until now, three-dimensional structural details of ZAP-70 have been completely unknown. In the absence of three-dimensional structural details for that protein, designing inhibitors based on that structure would have been impossible. We have now obtained crystals of a critical region of ZAP-70 containing its tandem SH2 domains, with and without bound ligands of various types, and have determined its three dimensional structure. With this information, it is now possible for the first time to rationally design inhibitors of ZAP-70 which can function as immunosuppressive agents, e.g. compounds which inhibit molecular interactions involving one or both of the ZAP-70 SH2 domains. Although the three-dimensional structures for several individual SH2 domains of other proteins are known, no one has heretofore reported determining the three-dimensional structure of a tandem SH2 region. And, as we discuss below, the three-dimensional coordinates of previously known SH2 domains would have been insufficient to solve the structure of the ZAP-70 tandem SH2 region.